This invention relates to a server, a data caching method, and a communication system, and more particularly, to a server, a data caching method, and a communication system, for sending encoded data to a plurality of transmission nodes in case where a user terminal is a mobile terminal.
Hitherto, in a wireless network, each base station caches a plurality of files. When a user terminal requests one file, if the file has been cached by a base station (also referred to as local base station) belonging to a scope in which the user terminal is located, the local base station directly provides a service to the user terminal. On the other hand, if the file has not been cached by the local base station, the local base station acquires the file data from a network and sends the acquired file data to the user terminal (refer to US 2005/0102300 A1). In US 2005/0102300 A1, the base station is required to cache the entire content of the file, causing the consumption of the storage area of the base station.
Moreover, in EP 2202888 A1, there is disclosed “Raptor code”, which is a typical implementation of fountain codes, which generate approximate infinite encoded packets from limited original packets, achieving a low encoding and decoding complexity. However, EP 2202888 A1 only proposes the application of fountain codes in data transmission in order to improve the efficiency and reliability of the data transmission, and an improvement of the storage efficiency by utilizing the fountain code to encode and store the data is not taken into consideration.
Further, in WO 2006/121900 A2, there are disclosed a multiple source wireless communication system and method. In the multiple source wireless communication system and method, in a cellular network, when a user terminal can receive communications from a plurality of base stations, Reed-Solomon coding (RS coding) or rateless encoding such as Tornado encoding or Raptor encoding is employed to encode a source data block including M packets, N packets of different subsets are sent from each of the base stations, the packets are received from the plurality of base stations at the receiver, and the source data block can be derived based on K (K=M+A) out of the N packets, where A<M, M<N.
In WO 2006/121900 A2, the wireless transmission terminals considered in the network are only confined to base stations. In the future, however, there are relay stations, small cells, and micro cells, etc. besides base stations in one cellar system. In addition, there is a macro cell containing a plurality of cells. Therefore, a problem of how to utilize different types of transmission terminals (including a case of overlapping coverage) to provide mobile reception for the user terminal has not been considered.
In addition, in WO 2006/121900 A2, applying the gain produced by mobile download of the user terminal to the optimization of data caching in the plurality of transmission sources is not taken into consideration. In fact, the mobility of the user terminal makes the same user terminal be able to receive the data from the plurality of base stations, and, for one base station, the user terminal can download the data from the one base station only in the period from moving into the coverage area of the one base station to moving out of the coverage area of the one base station.
When the cached data in a base station is more than the data that can be downloaded by the user terminal in the period, part of the data cannot be downloaded, which wastes the storage area in vain and reduces the data cache efficiency.
On the other hand, when the cached data in a base station is fewer, when the user terminal passes through the coverage area of the base station, the data is downloaded only in part of the period and the rest of the time is spare, causing the waste of the time. Besides, if the user terminal moves out of the coverage area of the base station in the period of downloading the data from the server via the base station, the case in which the download cannot be continued occurs, and on the contrary, the download efficiency of the user terminal is reduced.